Wind transportation Flashcards
Name three ways that sand can be transported by wind
Surface creep
Saltation
Suspension
Describe wind deposition
Deposition takes place when wind velocity falls below critical value. Velocity vairs according to size of sand particle.
Where is sand deposited?
In sheltered areas protected from the wind. leeward side of rocks eg
Whats is surface creep?
When larger sand particles are rolled along the desert floor
Whats is Saltation?
Sand particles move in a series of leaps as they are picked up by a gust of wind before being dropped downwind
What is Suspension?
fine sands and clays may be picked up and carried considerable distances. Common when high velocity winds create sandstorms
Give me an example of suspension
Sand form the Sahara transported to Florida, >8000 km away.
When is a sand dune NOT solely a depositional process?
when it is mobile and therefore a type of flow- an ongoing surface creep
When is a sand dune a depositional process?
when it is fixed and largely static it is truly depositional and represents a temporary store in balance at the time but vulnerable to change if environmental conditions change.
Define mechanical or physical weathering
Gradual disintegration of rocks without chemical change.
What is the most important type of mechanical weathering?
Thermal fracture in a desert environment.
What is thermal fracture?
ntense temperatures fluctuations cause rock to expand and contract. Moisture (from dew) promotes this process.
What is thermal fracture dependent on?
Colour, lithology ( rock type) and geological structure of the rocks
Name 4 types of Thermal fracture
granular disintegration
block separation
shattering
exfoliation
What is granular disintegration
granular rocks tend to crumble into individual grains of sand. the coloured minerals in granite are heated at different rates which facilitates the break of rocks
What two minerals are commonly found in granite?
black Mica and white quartz
Which types of rock are prone to granular disintegration?
granite and sandstone
What is block separation?
if rocks have a clear pattern of joints and bedding plains they are prone to breaking into blocks
Which types of rock are prone to block separation?
Sedimentary rocks such as limestone
What is shattering?
rocks that do not have separate grains or joints tend to shatter to form angular fragments.
Which type of rock is prone to shattering?
Igneous rock such as basalt
What is exfoliation?
The outer skin of the rock peeling away as a result of repeated heating and cooling. Rocks are poor heat conductors.
Which types of rock are prone to exfoliation?
Exposed granite or sandstone
What is salt crystallisation?
Rain dissolves salts in the soil. The high rate of evaporation draws water to the surface by capillary action and salt crystals are deposited on the ground surface. over time these crystals may grow and expand causing stresses that eventually break up rocks.
What is frost shattering.
Water often in the form of dew freezes and expands in cracks and pores in the rocks. Repeated cycles cause the rocks to shatter.
Where does frost shattering occur?
Temperature must be below 0 degrees centigrade. Common in upland or mountainous regions - middle east and northern fringes of Sahara desert.
What is chemical weathering?
Rocks that change as a result of chemical action (usually in the presence of water).
What indicates chemical weathering in hot deserts?
flaking or pitting of rocks. underground deposits of rock salt as can be found in the Atacama desert.
Which chemical weathering process is most effective in cold climates?
carbonation - the dissolving of limestone by acidic rain water
Why is weathering more intense in high temperatures?
because some chemical reactions are enhanced by heat
What are the main sources of water in deserts?
Exogenous rivers
Endoreic rivers
Ephemeral rivers
Episodic flash floods
What are exogenous rivers?
Rivers that have their source outside the desert region. they have suffucuent water to flow despite high rate of evaporation
Name some exogenous rivers?
Colorado, Indus, Nile
What are endoreic rivers?
rivers that flow into a desert but terminate in a lake of inland sea
Name an endoreic river
River Jordan which drains into the Dead Sea
What is an ephemeral river?
rivers or streams that flow intermittently. they may flow after storm event or fed by snowmelt. They can be powerful sources of erosion and flow rates can vary dramatically.
What are episodic flash floods?
infrequent rainfall events that involve torrential convectional storms releasing large amounts of water in a short space of time.water can flow as a sheet over the sun baked land ( sheet flooding) or can be confined in a channel (channel flash flooding)
What is one result of episodic flash floods?
large amounts of sediment ar washed out of the mountains to be deposited as vast alluvial plains on the lowlands below.
name 6 causes of desertification
over-cultivation
overgrazing
over irrigation
population increase
firewood
tourism
explain how overcultivation leads to desertification
intensive farming decreases soil fertility. lack of organic matter makes soil more likely to be washed or blown away
explain how overgrazing leads to desertification
if the land exceeds its carrying capacity the vegetation and soil will suffer
explain how overirrigation leads to desertification
if plants are over irrigated the salts can be dissolved in the soil and form a salty crust when the water evaporates ( salinisation). This created impermeable, infertile soil.
explain how population increase leads to desertification
if the rate of population increase exceeds the rate of food production it puts additional pressure on the land to be productive.
explain how firewood leads to desertification
land quickly becomes stripped of the few trees as people rely on firewood for cooking. This exposes the soil to wind and rain leading to soil erosion.
explain how tourism leads to desertification
high concentration so safari buses in countries such as Kenya leads to damage of vegetation, leading to soil erosion
What is the population of Africa’s Sahel region and how does it changeover time?
it is estimated at 260 million and doubles every 20 years. this is a 3% growth per year.
How does desertification affect ecosystems?
Loss of nutrients - due to over exploitation by agriculture eaves soils impoverished –> loss of biodiversity
Loss of topsoil - the most nutritious horizon, due to wind and rain
Increased salinity if irrigation is used
Water sources become depleted - plants and animals die
As plants die animals have to migrate to find other suitable veegetation
